The present invention is concerned with a method for the casting of hollow slabs out of concrete by slide-casting, whereat concrete mix is extruded onto a base by using one or several forming members forming the cavities and the mix is compacted by moving the forming member. The invention is also concerned with a slide-casting machine for casting hollow slabs out of concrete, which device comprises a deck plate, side walls, one or several feeder members for feeding the concrete mix, as well as one or several movable forming members for forming the cavities. The invention is in particular suitable for the production of prestressed hollow slabs. It may also be applied to the manufacture of hollow slabs of reinforced concrete.
Several slide-casting machines for hollow slabs are known in prior art, which are of a similar principle as compared with each other and in which the concrete mix is extruded in the machine by means of spiral screws. The machine runs along rails placed on the base. The spiral screw is of conical shape with the cone expanding towards the final end, whereby an efficient compacting of the concrete is also achieved.
Immediately as an extension of the spiral screw, there is a shaping member, i.e. a so-called cavity mandrel, which is vibrated by means of a vibrator fitted inside the mandrel. Moreover, a vibrator beam fitted in the deck portion of the machine is vibrated, whereat the vibration of the cavity mandrels together with the surface vibration at the top of the machine produces an ultimate compacting of the concrete.
The cavity mandrel is followed by a so-called follower tube, whose function is to support the cavity wall at the final end of the machine.
Drawbacks of the cavity mandrel are the strong noise (higher than 85 dBA) resulting from the high vibration frequency, the high power requirement, and the low efficiency of the vibration power used for the vibration.
By means of the present invention, the prior-art cavity vibration is replaced by using a compacting process suitable for compacting a soil-moist concrete mix.
The method in accordance with the present invention is characterized in that one end or both ends of the forming member are moved along a path of movement of desired shape. Most appropriately, one point of the longitudinal axis of the forming member maintains its position relative its support member. The slide-casting machine in accordance with the invention is characterized in that one end or both ends of the forming member can be moved along a path of movement of desired shape. The forming member may be attached to its support shaft by means of a universal-joint fastening.
In front of each forming member, there may be a screw spiral as the feeder member. Most appropriately, at least the initial end of the mandrel is moved. Within the path of movement of the initial end of the cavity mandrel, the stroke length of the mandrel is a few millimeters. At the same time, the mandrel may additionally either revolve around its longitudinal axis, or it may not revolve. The path of movement of the end of the mandrel may be of circular shape, but it may also be of some other shape, e.g. square.
When a mandrel revolving around its longitudinal axis is used, usually, cavities of circular section are produced in the hollow slabs. When the mandrel does not revolve around its longitudinal axis, the cross-sectional form of the mandrel may also be different from circular. In this way, the cavities can be shaped as desired. Even when a revolving mandrel is used, according to the present invention, it is possible to produce cavities of a sectional form different from circular if the path of movement of the end of the mandrel is not circular.
Advantages of the method in accordance with the invention include:
essentially lower noise level as compared with cavity vibrators whose vibration frequency is 150 to 250 Hz.
Owing to the wide path of movement of the end of the mandrel next to the spiral screw, the compacting process of the concrete can be shifted from the area of the screws to the area of the mandrel.